A phosphor is used for a vacuum fluorescent display (VFD), a field emission display (FED), a plasma display panel (PDP), a light emitting display (LED), or the like. To make a phosphor emit light, energy for rendering the phosphor excited is supplied to the phosphor, and the phosphor is excited by an excitation source having high energy, for example, vacuum ultraviolet rays, ultraviolet rays, electron beams, and blue light. However, since the phosphor is deformed by these excitation sources and thus tends to involve a decrease in luminance and to deteriorate, a phosphor having less luminance degradation is required. Accordingly, sialon phosphors, which show a slow decay in luminance, are introduced instead of silicate phosphors, phosphate phosphors, aluminate phosphors, and sulfide phosphors, conventionally used.
A sialon phosphor is a kind of acid nitrides having Si, Al, O, and N and includes an α-sialon phosphor and a β-sialon phosphor which have different crystal structures. Non-patent Literature 1 discloses an α-sialon phosphor, while Patent Documents 1, 2, 3, and 4 disclose an α-sialon phosphor and a light emitting device using the same. Also, Patent Document 5 discloses a β-sialon phosphor, while Patent Document 6 discloses a β-sialon phosphor and a light emitting device using the same.    [Non-patent Literature 1] J. W. H. vankrebel “On new rare earth doped M-Si-Al-O—N materials,” Tu Eindhoven The Netherland, P145-161(1998)    [Patent Document 1] JP Patent Publication No. 2002-363554    [Patent Document 2] JP Patent Publication No. 2003-336059    [Patent Document 3] JP Patent Publication No. 2004-238505    [Patent Document 4] JP Patent Publication No. 2007-31201    [Patent Document 5] JP Patent Publication No. S60-206889    [Patent Document 6] JP Patent Publication No. 2005-255895
α-sialon is a crystal structure having a unit structure represented by Si12-(m+n)Al(m+n)OnN8-n, in which two empty spheres are present. Metal ions having a relatively small radius, for example, Ca2+, may be engaged in the empty spheres of the crystal structure, and α-sialon engaging the metal ions may be represented by a general formula Mm/vSi12-(m+n)Al(m+n)OnN8-n:Eu, wherein M is a metal ion and V is a valence thereof. As stated in Non-patent Literature 1 and Patent Document 1, α-sialon engaging Ca and an active material Eu is known as a phosphor emitting light in a yellow region. Since this phosphor has a continuous excitation band from an ultraviolet region to a blue region and thus emits yellow light by irritation of ultraviolet rays or blue light, the phosphor may be used as a yellow phosphor for a white light emitting device.
This phosphor may be prepared by mixing proper amounts of precursor materials obtained from powders of silicon nitride, aluminum nitride, calcium carbonate (CaCO3), and europium oxide as starting materials and sintering the mixture at a high temperature in a nitrogen atmosphere. Further, a substrate of a high purity material with a limited amount of impurities disclosed in Patent Document 3 or metal silicone disclosed in Patent Document 4 are used so as to achieve high luminance.
Meanwhile, β-sialon is represented by a general formula Si6-xAlxOxN6-x. Patent Documents 5 and 6 disclose a β-sialon phosphor prepared by adding an active material to β-sialon. Patent Document 5 discloses a β-sialon phosphor obtained using μ-sialon and an active material, for example, Cu, Ag, or a rare-earth element such as Eu. However, it is reported that a Eu-activated β-sialon phosphor of Patent Document 5 emits light in a blue wavelength region from 410 nm to 440 nm, while a phosphor disclosed in Patent Document 6 is a green phosphor. Difference in emission color between the phosphors is probably due to a fact, as mentioned in Patent Document 6, that the active material Eu is not securely engaged in β-sialon since the Eu-activated β-sialon of Patent Document 5 has a low sintering temperature.
A Eu-activated β-sialon phosphor of Patent Document 6 emits green light and is excited by light in a blue wavelength region. Thus, the Eu-activated β-sialon phosphor attracts attention as a green light emitting phosphor for a white light emitting device, which is constituted by a blue light emitting device and a phosphor or by an ultraviolet light emitting device and a phosphor. In particular, the Eu-activated β-sialon phosphor has a narrow spectrum width of about 55 nm and good color purity and thus is expected to be used as a green phosphor for a white light emitting device requiring color reproducibility. However, since the Eu-activated β-sialon phosphor does not have sufficiently high luminance, enhancement of luminance is necessary.
A β-sialon phosphor is also prepared by mixing proper amounts of materials obtained from powders of silicon nitride, aluminum nitride, and an active material as starting materials and sintering the mixture at a high temperature in a nitrogen atmosphere. However, a β-sialon phosphor obtained by a currently known method using nitrides, such as silicon nitride or aluminum nitride, as a starting material does not have a sufficiently high luminance.
That is, in a conventional method of synthesizing a rare-earth element added β-sialon phosphor, raw materials including oxides and nitrides, such as Si3N4, SiO2, AIN, Al2O3, and Eu2O3, are mixed, and synthesized at 1,900° C. or higher in a nitrogen atmosphere. However, when β-sialon is synthesized by mixing a rare-earth element used as a bivalent cation activator in mixing the raw materials, other cations than Si and Al, which form sialon, may serve as impurities, degrading crystallinity of β-sialon, which may cause a decrease in luminance of the phosphor.
Further, as described above, although a yellow YAG phosphor is applied to a light emitting device to realize a white light emitting device for the first time, the white light emitting device shows a low color rendering index (CRI) as compared with a common lamp. Thus, a white light emitting device having an improved CRI by using green and red phosphors has been recently developed. As a green phosphor applied to the device, a silicate phosphor or a sulfide phosphor is used. However, such phosphors exhibit low high-temperature, thermal, and chemical stabilities, and thus a phosphor using nitrides is vigorously studied. Since a nitride phosphor is obtained by adding an activator to a host material, such as Si3N4 and sialon used for a high-temperature structure materials, the phosphor has superior color reproducibility and reliability for display and excellent high-temperature light emitting characteristics and thus is applied to a TV backlight and a lighting lamp. However, this phosphor has 70% or less of efficiency than the YAG phosphor, and thus improvement in efficiency is needed.